This invention relates to heating means for the inlet air flow to an air-breathing internal combustion engine, particularly a compression ignition engine.
In the operation of internal combustion engines, it is sometimes required to preheat the inlet airflow when operating in low ambient temperatures. In compression ignition engines in particular, a low air inlet temperature can cause difficulties when starting the engine and when the engine is running at idling speeds. It is therefore known to provide an electrical preheater in the form of a resistance coil suspended in the inlet air flow path, usually as part of a screwed plug that can be fixed via a threaded boss. Typically, this will be positioned on the engine air inlet manifold, or similarly close to the cylinder inlets so that the heating energy is not dissipated before the airflow enters the engine cylinders. Such heating means are known, for example, from UK Pat. Nos. 921986 and 933331.
With these known arrangements, it is rarely if ever that the coil is inspected to ensure that it has remained securely in place. But should the coil break loose and reach the entry to a combustion chamber, it can cause catastrophic damage to the engine. The possibility of detachment, e.g. due to fatigue failure, is increased by the operating conditions: of necessity the coil is subjected to repeated heating and cooling and to vibration both due to engine vibration and to aerodynamic forces since it must be placed where it is fully exposed to the air inlet flow. If an electrical heating resistance element is to respond quickly and efficiently, it must have a relatively small cross-section so that its operation would be adversely affected by any attempt to avoid this problem by providing a substantially more robust coil construction.
Other engine inlet heating means are known in which an electrical resistance member is clamped rigidly to a firm support (UK Pat. No. 340719) or contained in a rigid outer casing (UK Pat. Nos. 150605 and 180463) but although these measures avoid the dangers mentioned in the preceding paragraphs, the heating efficiency is considerably reduced because a much larger mass must be warmed before the inlet flow experiences any heating effect. Such loss of efficiency is particularly disadvantageous if the heating means are to be used as a cold-starting aid.
It is also known to support an inlet heating coil so that the coil axis extends in the direction of the inlet flow (UK Pat. Nos. 130370, 280555, 535175 and 555115). Such an arrangement considerably reduces the aerodynamic forces on the coil and so lessens the risk of detachment, but in this case also there can be an unacceptable loss of heating efficiency, in particular because a far smaller proportion of the flow can be influenced by the heated coil. Moreover, if such a coil is to heat the inlet flow to a number of cylinders of a piston engine, the entry temperature into the individual cylinders may then be markedly non-uniform.